Method of analyzing and controlling etchant solution concentrations

ABSTRACT

A METHOD AND APPARATUS FOR THE AUTOMATIC MONITORING, ANALYZING AND CONTROLLING OF THE ETCH RATE AND CHEMICAL CONCENTRATION OF A CHEMICAL MILLING SOLUTION RESPONSIVE TO PH VALUES AT FIRST AND SECOND EQUIVALENCE POINTS FOR BOTH CAUSTIC SODA AND INHIBITOR.

y Oct. 17, 1972 E, J, B|ELEFE| D IR` E TAL 3,698,867

METHOD OF ANALYZING AND OONTROILING ETCHANT SOLUTION OONCENTRATI NOriginal Filed Oct. 9, 1968` 2 Sheets-Sheet l #mmm wmmwr h6 .S

Q ma Oct. 17, 1972 E. J, BIELEFELD. JR.. ETAL 3,693,357

METHOD OF ANALYZING ND CONTROLL1NG ETCHANT SOLUTION CONCENTRATION 9,1968 2 Sheets-Sheet 2 Original Filed Oct.

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w v Il fx EVA Hh www ,V L www QQm. kmh v United States Patent 3,698,867METHOD OF ANALYZING AND CONTROLLING ETCHANT SOLUTION CONCENTRATIONSEwald J. Bielefeld, Jr., Benbrook, and Maurice J. Puma,

Fort Worth, Tex., assignors to General Dynamics Corporation Originalapplication Oct. 9, 1968, Ser. No. 766,161. Divided and this applicationMar. 29, 1971, Ser. No. 129,257

Int. Cl. C23f 1/08; G0511 11/08; G01n 31/16 U.S. Cl. 23-230 A 3 ClaimsABSTRACT F THE DISCLOSURE A method and apparatus for the automaticmonitoring, analyzing and controlling of the etch rate and chemicalconcentration of a chemical milling solution responsive to pH values atirst and second equivalence points for both caustic soda and inhibitor.

This is a division of our co-pending application Ser. No. 766,161, tiledOct. 9, 1968 now U.S. Pat. No. 3,607,549.

The invention herein described was made in the course of or under acontract or subcontract thereunder, with the Department of the AirForce.

The present invention relates generally to a method and apparatus forautomatically sampling, analyzing and. controlling chemical millingsolutions.

More particularly this invention is directed to a process and relatedinstrumentation for sampling, analyzing and controlling the relativestrength of aluminum chemical milling solutions which involvessimultaneous titration of two sample portions to bring each near one 'ofthe two equivalence points.

PRIOR ART In the past, etchant solution analyses have been performed bymanually sampling and analyzing the etchant solution. Results of theanalysis required manual computation to determine necessary correctiveaction-such as tank drain-off or replenishment of depleted chemicalsbothof which corrections were also, of necessity, performed manually.

Several disadvantages are inherent in this prior art operation which thepresent invention obviates. The manual method is time consuming, and thelag time between analysis and corrective action prevents the maintenanceof the precise concentrations necessary for a constant and uniform etchrate. The invention also permits the elimination of human errors andprevents interruption of production during the period in whichcorrective action is being taken.

BRIEF SUMMARY OF THE INVENTION This invention affords an automaticchemical sampler, analyzer and controller which provides a method andmeans by which chemical milling solutions may be automaticallymonitored, analyzed and controlled during all phases of chemical millingor etching through a coordination and metering of material pumps and pHmeters which are set at desired, predetermined values, variances fromthese values beyond predetermined critical limits at either or bothequivalence points initiating signals which in turn activate controlvalves at the chemical milling tanks, corrective action being takenconstantly and automatically, thereby maintaining constant chemicalconcentrations and etch rate, which in context of the often criticaldegree of etch in present applications is of prime importance.

The invention will be readily understood by those versed in the art whentaken in context of the following specification and related drawings,wherein:

3,698,867 Patented Oct. 17, 1972 rice PIG. 1 is a typical titrationcurve for an aluminum etch tank showing the two equivalence pointsresulting from the neutralization of the sample with a standardized acidsolution; and

FIG. 2 is a schematic plan view of a typical production tank constructedin accordance with the present invention and showing the relationship ofthe various components to each other and to the system.

The process for carrying out the invention is applicable tosubstantially all solutions of this type, however, as an exemplary andpreferred process the analysis of caustic soda and sodium aluminate isdescribed herein since it is the most common yprocess employed by theindustry.

Referring first to FIG. 1, the analysis of N1 (caustic soda) and N2(sodium aluminate) may be described briefly as follows:

A 5 milliliter sample of etchant is taken and neutralized to a pH of11.5 with one normal acid. The number of milliliters of one normal acidrequired to reach a pH of 11.5 on a 5 milliliter sample is defined to bethe N1 value, FIG. 1, and is a measure of the free caustic soda. Uponadding more acid the sodium aluminate is converted to aluminum hydroxidewith another change in pH from 11.5 to 8.5. The number of milliliters ofone normal acid required to lower the pH from 11.5 to 8.5 on a 5milliliter sample is defined to be the N2 value and is a measure of theinhibitors (primarily sodium aluminate). The N1 and N2 equivalencepoints are shown in the titration curve 110.

Referring now to FIG. 2, a system constructed according to the presentinvention is shown as generally comprising an etch tank section 210 anda control section 212. Etch tank section 210, wherein the actual etchingis accomplished, is controlled by control section 212 which accomplishesthe pH analysis and performs the necessary corrective action based onthe analysis. If standard acid solution 214 and the etch tank solution216 are allowed to ilow via dual metering pumps 218, 220 at a constantrate, and mixed with water in the mixing chambers 252, 254, to avoidexcessive buildup of heat, a neutralization reaction occcurs. If theacid in the solution 214 is exactly equivalent to the caustic soda inthe etch solution 216, the resultant pH will be 11.5 as determined bythe titration curve, FIG. 1. A pH of less than 11.5 would mean an acidcondition or a caustic soda deficiency requiring corrective action,i.e., addition of caustic soda to the etchant solution 216 and isreadily detected by use of the present invention because the causticexhibits a sharp change in pH value at the lirst equivalence (N1) point.On the other hand, a pH of more than 11.5 would mean an alkalinecondition or an excess of caustic soda requiring no corrective action.Similar but opposite pH changes occur for the sodium aluminate(inhibitor) analysis with corrective action (drain off) being takenabove 8.5 pH, also readily detected because the inhibitor exhibits asharp change' in pH at the second equivalence (N2) point.

A typical cycle for an etch tank solution analyzer and controller isdescribed below. Although only one tank system is described in theinterest of clarity, it is apparent that any number may be controlled byexpanding the system. Briefly, prior to being electrically activated,the unit is set at the zero point on the stepping relay 222, FIG. 2.Relays 224, 226 are open, and all addition valves 228, drain valves 230and sample valves 232 are closed. The master switch activates both pHmeters 234, 236, master timer 238, and metering pumps 218, 220. Thewater liow is constant and is manually set at the water injection meters256, 258. The master timer 238 is preset to pulse stepping relay 222which in turn controls the entire sequence of operations. The oddnumbered contacts 1, 3, 5, 7 and 9 on stepping relay 222 are samplepurging steps where no pH meter signals are transmitted. The continuityof numbers on the stepping relay 222 indicates that it controls morethan one tank, however, since this speciiication shows the operation ofonly one tank, there are only two settings pertinent here, positions 1and 2. The zero point indicates the inoperative or shut off position.Master timer 238 is preset to hold stepping relay 222 on position number1 contact for suicient time to purge the solution ow system. Positionnumber 2 on stepping relay 222 activates relays allowing pH metersignals to pass to timers 240, 242, the former controlling additionvalve 228 and the latter drain valve 230, the pH meter signals beingresponsive to any variation in equivalence points for which the pHmeters are set. The dwell time on position number 2 contact is only longenough for the pH meter to signal the appropriate timers shouldcorrective action tbe necessary. Thus, each etch tank is analyzed andcorrective action taken in sequence, automatically and continuously.

It should be noted that the water to mixing chambers 252, 254 isrequired to dissipate the heat of reaction and does not affect theresultant pH (hydrogen ion concentration).

The cycle begins as above noted, by purging control section 212, inwhich master timer 238 pulses stepping relay 222 to contact pointnumber 1. As seen in FIG. 2, sample solenoid valve 232 to etch tank 246is opened allowing a sample of etchant 216 from a continuously llowingetchant stream owing from the etch tank 246 through ow pump 248 tocontinuous drain 250, to be diverted through solenoid sample valve 232,which etchant is then pumped through metering pumps 218, 220 to mixingchambers 252, 254 Where the metered sample of etchant solution isthoroughly mixed with 36 normal acid (H2804) solution and diluted withwater controlled by water injection meters 256, 258 before passing to owchambers 260, 262 where the pH measurements are made. The sample andacid flow rates are preset in metering pumps 218, 220= which areadjusted to be compatible with the desired equivalence points asindicated at N1, N2 of FIG. l. Again referring to FIG. 2, metering pumps218, 220 pump etchant and standard acid solutions continuously while thesolution controller device is on and are not controlled or affected byany automatic action of the solution controller device. Concurrentlywith the above, when the solution purging period is complete and mastertimer 238,pulses stepping relay 222 to contact point 2, relays 224, 22'6are closed responsive to contact point 2. This completes the circuitsfrom the pH meters 234, 236 to the timers 240, 242.

The pH meters 234, 236 read the pH of the solutions flowing through flowchambers 260', 2'62 continuously, but signals are not transmitted unlessthe pH is below the N1 or above the N2 set points. For example, the N1flow chamber 260 should have a pH reading above 11.5 to be in properconcentration range. An etch solution depleted in caustic soda will notuse all of the allotted acid, resulting in a lower pH reading whichindicates a more acid condition. Should the pH fall below 11.5 the N1 pHmeter 234 is preset to send an electrical signal through closed relay226 to timer 240 which in turn controls caustic soda addition valve 228to etch tank 246. This allows replenishment of caustic soda to etch tank246 for a preset time period and is independent of other action duringthe cycle. Similarly, the N2 pH meter 236 should have a pH reading ofless than 8.5 to be in proper range. A solution having a higher thandesired inhibiting content (sodium aluminate) will have a higher pH than8.5 since the increase of sodium aluminate will use up the neutralizingacid. A pH reading greater than the preset 8.5 will cause the N2 pHmeter 236 to send an electrical signal through relay 224 which in turnactivates timer 242 controlling drain valve 230 on etch tank 246. Thisallows etch solution 216 in etch tank 246 to drain for a preset time,thereby reducing the excess aluminum buildup. Timers 240, 242automatically reset for the next cycle and a constant level control 264is independently activated to allow addition of water through valve 266to etch tank 246.

At the completion of the rst tank analysis, master timer 238, whichdrives stepping relay 222, pulses stepping relay 222 to contact pointnumber 3. This allows relays 224 and 226 to open and sample valve 232 toclose. Sample valves are attached to each of the other tanks in thesystem (not shown) in the same manner in which sample valve 232 isattached to etch tank 246 and are opened sequentially so that thepurging, analysis and corrective action process for all tanks occur inthe same manner as that described for the etch tank 246 shown. When thecycle has completed for all tanks, stepping relay 222 resets and thecycle repeats itself.

In summary, the present invention for the rst time provides a method andits associated system for continuously, automatically analyzing andcontrolling etch solutions conssiting of a container having a constantlevel control for the solution to be analyzed, and which includes ameans of adding ingredients to or removing them from the containersolution. The solution is automatically sampled responsive to an impulsefrom a stepping relay, treated with a standard solution and pumpedthrough ilow chambers where the pH value is measured for both causticand inhibitor simultaneously and independently, the pH value thenactivating timer means to add caustic or delete inhibitor in thecontainer in accordance with and responsive to the readings thusderived.

We claim:

1. The method for automatically analyzing and controlling theconcentration of an etchant solution which includes caustic andinhibitor comprising the steps of:

(A) determining the desired chemical concentration of the etchantsolution and the critical limits thereof based on rst and secondequivalence points of a standard etchant solution;

(B) causing the etchant solution to be periodically and automaticallysampled;

(C) causing the etchant solution sample to be equally distributed to rstand second mixing chambers respectively, and in said rst mixing chambermixing one etchant solution sample portion with a predetermined amountof standard neutralizing solution corresponding to the rst equivalencepoint of the standard etchant solution, in said second mixing chambermixing the other etchant solution portion with a second predeterminedamount of standard neutralizing solution corresponding to the secondequivalence point of the standard etchant solution, and sensing theproximity of the mixed portions to the rst and second equivalence pointsrespectively;

(D) automatically determining from said sensing any deviation beyond therst and second equivalence point critical limits of the etchantconcentration;

(E) automatically making an adjustment to the etchant solutionconcentration responsive to said determined deviation, to therebymaintain the etchant solution concentration within said critical limits.

2. The method for controlling an etchant solution delined by claim 1wherein:

(A) said sampling comprises withdrawing etchant solution at a set flowrate and predetermined intervals by a solenoid activated meteringdevice;

(B) said mixing comprises withdrawing said neutralizing solutionsimultaneously at a predetermined rate via a metering device for eachetchant sample portion and causing the etchant sample portions andneutralizing solutions to mix and the sensing step cornprises sensingthe hydrogen ion concentration resultant from said mixing step;

(C) said determination step comprises generating a signal whichcorresponds to the degree of variation in caustic or inhibitor of saidsolution from said desired concentration outside said predeterminedequivalence point critical limits;

(D) said adjustment comprises selectively adding chemical to the etchantsolution where said determination step indicates depletion of caustic ordraining the etchant solution in the work tank where said determinationstep indicates an excess of inhibitor responsive to the result of saiddetermination step.

3. The method dened in claim 1 wherein said automatic analyzing andcontrolling of an etchant solution comprises the steps of:

(A) activating a solenoid by pulses from a timer actuated stepping relaywhich has been preset to desired time intervals, thereby allowingwithdrawal at the proper flow rate of the etchant solution sample via ametering device;

(B) testing the said mixed solution portions for hydrogen ionconcentration simultaneously and independently with plural pH meters todetermine the References Cited UNITED STATES PATENTS 9/1928 Smith et al23--230 A 2/1953 Sheen 23-253 R 3/ 1961 Quittner 23--253 A 7/ 1965Nicholson 23-230 A MORRIS O. WOLK, Primary Examiner D. G. MILLMAN,Assistant Examiner U.S. Cl. X.R.

